This invention relates to an optical resin, a process for producing the same, and a molded article obtained therefrom.
Methacrylic resins including methyl methacrylate as a major component are in general excellent in transparency and weather resistance, and thus are widely used in various fields such as automobile parts, electric parts, optical elements, etc. But these methacrylic resins have a heat deformation temperature of about 100.degree. C. at highest, and are not always sufficient in heat resistance. Therefore, improvement in heat resistance has strongly been desired.
In order to improve the heat resistance of methacrylic resins, there have been proposed a process for copolymerizing methyl methacrylate with .alpha.-methylstyrene and maleic anhydride (Japanese Patent Examined Publication No. 49-10156), a process for copolymerizing methyl methacrylate with a polyfunctional monomer, followed by crosslinking (Japanese Patent Unexamined Publication No. 63-30510), and a process for producing a methacrylimide-containing polymer by reacting a methacrylic resin with a primary amine (Japanese Patent Unexamined Publication No. 61-64703). According to these proposals, the heat resistance is improved to some extent, but there are problems in that the productivity is low and complicated due to extremely slow polymerization rate and the use of special reaction apparatus, and properties are not good due to production of remarkably colored molded articles and poor processability. Further, it is also proposed to copolymerize methyl methacrylate with N-arylmaleimide (Japanese Patent Examined Publication No. 43-9753), but the heat resistance is not improved effectively and the productivity is still insufficient.
On the other hand, copolymers of methyl methacrylate and an N-substituted maleimide can be produced by bulk polymerization, solution polymerization, emulsion polymerization, suspension polymerization, and the like. Among these polymerization methods, the bulk polymerization method has problems in that a reaction apparatus is expensive and control of the reaction is difficult. The solution polymerization method has a defect in that productivity is low. The emulsion polymerization method and the suspension polymerization method have advantages over the bulk polymerization method and the solution polymerization method in that the reaction apparatus is not expensive, control of the reaction is easy and productivity is high. But, according to the emulsion polymerization method, there are some problems in that an emulsifying agent is mixed into a final product due to the use of a large amount of it, resulting in generation of opaqueness and making the transparency remarkably poor. Therefore, the emulsion polymerization method is not suitable for producing an optical resin.
Further, according to a known suspension polymerization, the resulting copolymer of methyl methacrylate and N-substituted maleimide is undesirably poor in transparency which is a feature of methacrylic resin containing no N-substituted maleimide. This is caused by a remarkably large amount of unreacted N-substituted maleimide (residual monomer) and hydrolyzed products of N-substituted maleimide.